Multi-stage combustors are used particularly in lean burn fuel systems of gas turbine engines to reduce unwanted emissions while maintaining thermal efficiency and flame stability. For example, duplex fuel injectors have pilot and mains fuel manifolds feeding pilot and mains discharge orifices of the injectors. At low power conditions only the pilot stage is activated, while at higher power conditions both pilot and mains stages are activated. The fuel for the manifolds typically derives from a pumped and metered supply. A splitter valve can then be provided to selectively split the metered supply between the manifolds as required for a given staging.
A typical annular combustor has a circumferential arrangement of fuel injectors, each associated with respective pilot and mains feeds extending from the circumferentially extending pilot and mains manifolds. Each injector has a nozzle housing the discharge orifices which discharge fuel into the combustion chamber of the combustor, a feed arm for the transport of fuel to the nozzle, and a head at the outside of the combustor at which the pilot and mains feeds enter the feed arm. Within the injectors, a check/distribution valve, known as a fuel flow scheduling valve (FSV), is typically associated with each feed so that when a pilot or mains stage is de-selected, the valve (i) provides a drip tight seal preventing fuel from leaking into the injector causing coking and (ii) prevents combustion chamber gases entering the fuel system.
Multi-stage combustors may have further stages and/or manifolds. For example, the pilot manifold may be split into two manifolds for lean blow-out prevention.
During pilot-only operation, the splitter valve generally directs fuel for burning flows only through the pilot fuel circuit (i.e. pilot manifold and feeds). It is therefore conventional to control temperatures in the stagnant (i.e. mains) fuel circuit to prevent coking due to heat pick up from the hot engine casing. One known approach, for example described in EP A 2469057 (hereby incorporated by reference), is to provide a separate recirculation manifold which is used to keep the fuel in the mains manifold cool when it is deselected. It does this by keeping the fuel in the mains manifold moving, allowing flow from a high pressure source (typically a gear pump delivery pressure, HP) to pass through a cooling flow solenoid valve, through the recirculation and mains manifolds before returning to a low pressure sink (typically a gear pump inlet pressure LP) via a valve and restrictor network. The recirculation and mains manifolds experience a low intermediate pressure, above LP but insufficient to crack open the mains FSVs. When mains is selected, a cooling flow also has to be maintained in the recirculation manifold to avoid coking.
A problem associated with this approach is that blockage may occur in the recirculation path. The consequence of such a failure is dependent on the location of the blockage.
For example, if the blockage occurs in the recirculation path on the return-to-LP side of the manifolds, the result can be an increased pressure in the recirculation path which opens one or more of the mains FSVs, potentially causing hot streaks due to the resultant mal-distribution of fuel flow and, as a consequence, turbine damage.
If the blockage occurs on the recirculation path on the HP feed side of the manifolds, the result can be a loss of cooling flow and/or pressure in the recirculation path at the injectors, potentially resulting in combustion gases leaking back from combustion chamber pressure, past the mains FSVs and thence to the low pressure side fuel system of the system via the exit from the recirculation path. This can lead to damage and/or failure within the fuel system. Air flow past the fuel drip tight seal of an FSV may be possible due to the low viscosity of air relative to that of fuel.
A further possible failure mode with this system is associated with the relatively high cracking pressure of the FSVs, set to avoid incorrect opening of the mains FSVs in pilot-only operation at conditions where the recirculation path sink pressure, LP, can be high relative to the combustion chamber. If one of the mains FSVs fails open, significant mal-distribution of fuel injection around the combustor can persist since flow through the failed open mains FSV has to increase to a significant level before other mains FSVs fully open. This can result in turbine torching and consequent damage.